Bruno Cadot* received the “Elsevier Big Prize” at the WMS Congress (2-6 October 2018 in Mendoza, Argentina) for his oral presentation on nucleus-cytoskeleton interactions and nuclear positioning during muscle development.
For which projects did you receive this award?
At this congress, I presented the last ten years of research that I have accomplished at the Institute of Myology. The initial idea was to study the positioning of the nuclei during muscle fibre differentiation. This phenomenon has not really been studied whereas the position of the nucleus is something that pathologists use all the time when looking at muscle sections; it serves as a marker to know if the muscle is in regeneration, or in patients if there is a problem. But until then, few researchers were interested in understanding whether the position of the nucleus was related to the pathology or function of the muscle itself.
For the past 10 years, we have been developing cell systems that simulate events taking place in the organ and that can be observed with a microscopic. By using genetic modifications, biochemistry approaches, image analysis and computer simulations, we have been able to define four types of successive movements observed during muscle fibre formation and have determined the mechanisms of the first three. These are the results that I presented at the WMS.
Could you provide details of the mechanisms of these first 3 steps?
Muscle fibers, which contain many nuclei, result from the fusion of several mononucleated cells.
i) 1st movement: when a myoblast fuses with a cell that already has several nuclei (the “beginning” of muscle fiber or myotube), the myoblast nucleus will move from the fusion site to the center of the myotube, thanks to the cytoskeleton (microtubules carrying molecular motors).
ii) Then, the nuclei will spread more or less evenly inside the myotube, again, through the cytoskeleton, but in a different way using other molecular motors.
iii) Afterwards, as the contractile apparatus of the cell is being positioned, the myofibrils, long actin and myosin fibres arranged lengthways, will push the nuclei to the periphery of the cell by a contraction mechanism. When the cell begins to contract, desmin will gradually establish cross-links between the myofibrils and act as a zipper. The space where the nuclei are held will then shrink more and more, and the nucleus will be pushed out of the central space where all the myofibrils are and will have to deform to be able to pass.
In some diseases, the nuclei do not go to the periphery, or, if they go initially, they sometimes return to the centre, as in centronuclear myopathies; but we do not know why or how this occurs. We think that the rigidity of the nuclear envelope is important: if it is too rigid, the nucleus will not be able to pass between the myofibrils and will stay in the center, and if the nucleus is too soft, it will stay in the middle because it can be completely deformed between myofibrils. So we think that the rigidity of the nucleus is important for this movement.
Moreover, during their differentiation, the cells reorganise their microtubular cytoskeleton. Indeed, in the early stages, the nucleus becomes the centre of microtubule organisation, an activity normally devoted to the centrosome. This phenomenon described more than 30 years ago, has remained unexplained until now. We were able to identify a protein, Nesprin-1, bound to the outer membrane of the nucleus, that is required for anchoring microtubules to it. In the absence of the latter or in the case of a mutation giving rise to congenital dystrophy, the nuclei are no longer arranged equidistantly in muscle fibers. Its presence, and therefore the transfer of microtubule organisation to the nuclear envelope represents a major event for the formation of muscle fibers and for their function. This new role of Nesprin, thus far known to transmit mechanical signals to the nucleus, places it as a major player in cell organisation. Thanks to this discovery, we have been able to explain the role of this repositioning in the movement of nuclei because they use microtubules to distribute themselves along the muscle cells.
What is the mechanism of the 4th movement?
This is the grouping of some nuclei under the neuromuscular junction. When the nerve comes to touch the muscle fiber, without knowing precisely when it occurs during the different stages of differentiation, we observe that 6 to 8 nuclei are found under this part where the nerve meets the muscle. We have observed that if these nuclei are poorly positioned, there is poor signal transmission between the nerve and the muscle, and therefore poor contraction. I think that, as in the 2nd movement, it depends on the microtubules, because once the nuclei are in the periphery, they can still move longitudinally, but we still have to demonstrate it precisely.
* Researcher in the group “Physiopathology & therapy of autosomal dominant centronuclear myopathy” directed by Marc Bitoun
References :
• Gimpel P, Lee YL, Sobota RM, Calvi A, Koullourou V, Patel R, Mamchaoui K, Nédélec F, Shackleton S, Schmoranzer J, Burke B, Gomes ER, Cadot B. Nesprin-1α-Dependent Microtubule Nucleation from the Nuclear Envelope via Akap450 Is Necessary for Nuclear Positioning in Muscle Cells. Current Biology. 2017 Sep 27 pii: S0960-9822(17)31069-2. doi: 10.1016/j.cub.2017.08.031.
• Roman W, Martins JP, Carvalho FA, Voituriez R, Abella JVG, Santos NC, Cadot B, Way M, Gomes ER. Myofibril contraction and crosslinking drive nuclear movement to the periphery of skeletal muscle. Nature Cell Biology. 2017 Sep 11. doi: 10.1038/ncb3605.
• Gache V, Gomes ER, Cadot B. Molecular motors involved in nuclear movement during skeletal muscle differentiation. Molecular Biology of the Cell. April 1, 2017 vol. 28 no. 7 865-874.
• Cadot B, Gache V, Vasyutina E, Falcone S, Birchmeier C, Gomes ER. Nuclear movement during myotube formation is microtubule and dynein dependent and is regulated by Cdc42, Par6 and Par3. EMBO Rep. 2012 Aug 1;13(8):741-9.
• Metzger T, Gache V, Xu M, Cadot B, Folker E, Richardson BE, Gomes ER, Baylies MK. MAP and Kinesin dependent nuclear positioning is required for skeletal muscle function. Nature. 2012 Mar 18;484(7392):120-4.
